1. Field of the Invention
The present invention relates to a multiple-stage nozzle throat for increasing the burn rate of high speed solid propellant steering rocket motors that employ a progressive center perforate propellant grain configuration.
2. Description of the Prior Art
In a rocket motor having a center perforate solid propellant grain, the chamber pressure versus time history is generally progressive. That is to say, the pressure continues to increase throughout the time of the burn. This is because the area of the propellant burning surface continues to increase during burning. When no change occurs in the nozzle area during burning, the rocket motor Kn (surface area divided by nozzle throat area) progressively increases as the propellant is consumed. Thus, a typical chamber pressure versus time trace for center perforate rocket motors comprises a continual upward slope. This characteristic of center perforate rocket motors has been the source of a serious problem known as the "weight penalty" in the art because of the need for thicker and heavier structures for containing the resulting higher motor chamber pressures.
In order to obtain the delivery of a relatively constant pressure or thrust, the use of complex grain configurations or designs, such as star perforations, have been proposed. With an appropriate choice of such a complex perforation profile, it is possible to minimize the range of variation of the propellant surface area over the entire period of the burn thereby to maintain the motor Kn, and hence, the chamber pressure relatively constant. Such complex perforation profiles, however, have introduced problems in respect of achieving very high loading density and very short action time in high pressure rapid response steering rocket motors. There are, in addition, higher manufacturing costs related to tooling and propellant loading operations.
The available envelope size and the need for very high loading density in high pressure rapid response steering rocket motors precludes the effective use of star perforation propellant grains. Additionally, the need for short action time makes it critical that the motor reach and operate at its most efficient chamber pressure very quickly.
The present invention was devised to fill the technological gap that has existed in the art in these respects.